Simulation of the structure of dispersions of nanoparticles with induced magnetic dipoles

Results of computer simulation of dispersions of magnetized spherical nanoparticles carried out by the Monte Carlo method with the application of parallel calculations are presented. Equilibrium structures of magnetic dispersions with different degrees of volume filling and compositions (the addition of magnetic spheres with lower diameters or similar particles of nonmagnetic materials) formed under the action of a constant magnetic field are determined.     

Preparation and properties of colloidal iron dispersions

We systematically study the properties of dispersions of iron-based colloids synthesized in a broad size range by thermal decomposition of ironcarbonyl using different stabilizing surfactants. The synthesis results in stable dispersions of monodomain magnetic colloids. Our particles appear to consist of an amorphous Fe(0.75)C(0.25) alloy. Sizes of particles coated with modified polyisobutene or oleic acid can be easily controlled in the 2-10 nm range by varying the amounts of reactants. Extensive characterization with various techniques gives particle sizes that agree well with each other. In contrast to dispersions of small particles, which consist of single colloids, dynamic aggregates are present in dispersions of larger particles. On exposure to air, an oxide layer forms on the particle surface, consisting of a disordered Fe(III) oxide.     

Effects of structural transformations in magnetic emulsions

Structural transformations and relevant changes in the magnetic and optical properties of magnetosensitive emulsions based on magnetic fluids are experimentally studied. Peculiarities of the changes in the magnetic susceptibility of emulsions associated with the deformation of their microdroplets and the effect of phase inversion (the transformation of dispersions of magnetic droplets in nonmagnetic media into dispersions of nonmagnetic droplets in magnetic fluids) are established. Optical effects occurring in magnetic emulsions under the combined action of a shear flow and a magnetic field are studied. It is concluded that optically active composition media may be developed on the basis of magnetic fluids.     

Structural Stability,Electronic and Magnetic Properties of O-doped Monolayer C_2N

Based on the first-principles of density functional theory, the structural stability,electronic and magnetic properties of the O-doped monolayer C_2N are investigated. In details, the lattice parameters, band structures, density of states and phonon dispersions of O-doped monolayer C_2N are obtained and analyzed. Our results show that the introduction of oxygen dopants can cause a significant local lattice distortion. The band structure indicates that monolayer C_2N is a semiconductor with a direct band gap of 1.631 eV. The electronic properties of monolayer C_2N can be regulated by oxygen dopant atoms with different numbers. However, doping nonmetal oxygen element in monolayer C_2N does not affect its magnetic properties. In other words, the pure and O-doped systems are all nonmagnetic. The phonon dispersions of all the O-doped cases are found to have not any imaginary frequencies, which indicates that the structures of these O-doped systems have good structural stability.     

单晶Ni纳米线阵列的制备与磁性能

采用恒电流沉积方法, 在多孔阳极氧化铝(AAO)模板中制备出了具有单晶结构的Ni纳米线阵列. 采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)和X射线衍射(XRD)技术对制备的Ni纳米线阵列的形貌及结构进行了表征. 利用振动样品磁强计(VSM)对单晶Ni纳米线阵列的磁性能进行了研究. 结果表明, 单晶镍纳米线阵列的易磁化方向为纳米线轴向, 并且与多晶纳米线相比显示出了更高的矫顽力. 直径为30 nm的纳米线具有较高的矫顽力(8.236×104 A/m)和较高的剩磁比(Mr＝0.94Ms).     

Preparation and properties of poly(acrylic acid) oligomer stabilized superparamagnetic ferrofluid

Ferrofluids, which are stable dispersions of magnetic particles, behave as liquids that have strong magnetic properties. Nanoparticles of magnetite with a mean diameter of 10-15 nm, which are in the range of superparamagnetism, are usually prepared by the traditional method of co-precipitation from ferrous and ferric electrolyte solution. When diluted, the ferrofluid dispersions are not stable if anionic or cationic surfactants are used as the stabilizer. This work presents an efficient way to prepare a stable aqueous nanomagnetite dispersion. A stable ferrofluid containing Fe3O4 nanoparticles was synthesized via co-precipitation in the presence of poly(acrylic acid) oligomer. The mechanism, microstructure, and properties of the ferrofluid were investigated. The results indicate that the PAA oligomers promoted the nucleation and inhibited the growth of the magnetic iron oxide, and the average diameter of each individual Fe3O4 particle was smaller than 10 nm. In addition, the PAA oligomers provided both electrostatic and steric repulsion against particle aggregation, and the stability of dispersions could be controlled by adjusting the pH value of solution. A small amount of Fe2O3 was found in the nanoparticles but the superparamagnetic behavior of the nanoparticles was not affected.     

Structural transformations in magnetic suspensions

Structural transformations in dispersions of micron-sized iron particles suspended in a magnetite ferrofluid (the colloidal suspension of ferromagnetic nanoparticles in nonmagnetic liquid) are theoretically considered. An attempt is made to explain the tendency of iron particles to form doublets and longer chain aggregates with finite distance between particles in external magnetic field observed in recent experiments; in colloidal ferrofluid, micron-sized iron particles approach one another to finite distance that is equal approximately to the particle diameter. At moderate magnetic fields, minimal distance between approached particles is nearly independent of the strength of magnetic field. In ordinary magnetorheological dispersions, which are suspensions of magnetizing micron-sized particles in nonmagnetic liquid, the approach of particles practically does not occur up to their physical contact.     

Large-scale fabrication of periodic nanostructured materials by using hexagonal non-close-packed colloidal crystals as templates

This letter reports a versatile nonlithographic technique for mass fabricating three types of technologically important materials-polymer microwell arrays, 2D-ordered magnetic nanodots, and semiconductor nanopillar arrays, each with high crystalline qualities and wafer-scale sizes. Spin-coated hexagonal non-close-packed silica colloidal crystals embedded in a polymer matrix are used as starting templates to create 2D polymeric microwell arrays. These through-hole arrays can then be used as second-generation templates to make periodic magnetic nanodots and semiconductor nanopillars. This self-assembly approach is compatible with standard semiconductor microfabrication, and complex micropatterns can be created for potential device applications. The wafer-scale technique may find important applications in biomicroanalysis, high-density magnetic recording media, and microphotonics.     

Fabrication of large-area ferromagnetic arrays using etched nanosphere lithography

Nanosphere lithography (NSL) is a simple, cost-effective, and powerful technique capable of producing large-area arrays of ferromagnetic nanostructures with dimensions below 100 nm. These properties make NSL an attractive process for the fabrication of arrays of magnetic elements with applications in magnetic data storage. The main disadvantage with conventional NSL is that the monolayer of spheres always contains imperfections that are transferred to the resulting nanostructures. This can significantly affect the structural and magnetic properties of the fabricated array. In this paper we present a novel adaptation of NSL that reduces the effect of such defects on the resulting nanostructures. The technique also offers excellent control over the diameter, aspect ratio, and pitch of the fabricated elements. These properties are demonstrated through the fabrication of arrays of Ni elements of 210 nm diameter and arrays of Co elements with diameters between 200 and 320 nm.     

Light scattering by polyhedral particles in the Rayleigh-Gans domain

Summary The light scattering properties of certain regular polyhedral particles in theRayleigh-Gans region are considered. The particle scattering factors are derived for dispersions of square pyramids and dispersions of octahedra, oriented at random to the incident beam. Numerical results are presented both for the average scattering factor and for the average 45° dissymmetry of arrays of such particles.

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Zusammenfassung Es wurde die Lichtstreuung pyramidenf?rmiger und oktaedrischer Teilchen imRayleigh-Gans-Bereich untersucht und Ausdrücke für den Teilchen-Streufaktor bei statistischer Orientierung solcher Teilchen abgeleitet. Für den mittleren Teilchen-Streufaktor und für die mittlere Dissymmetrie wurden numerische Ergebnisse angegeben.

     

Self-Assembly Anisotropic Magnetic Nanowire Films Induced by External Magnetic Field

Self-assembly generated materials induced by an external magnetic field have attracted considerable interest following the development of nanodevices. However, the fabrication of macroscopic and anisotropic magnetic films at the nanoscale remains a challenge. Here, anisotropic magnetic films are successfully prepared using a solution-based nanowire assembly strategy under a magnetic field. The assembly process is manipulated by changing the thickness of silica shell coated on the surface of magnetic nanowires. The anisotropic magnetic films show highly anisotropic magnetization under different angles of magnetic field and better magnetization properties than that of disordered magnetic films. The well-defined nanowire arrays enable magnetization anisotropic property which may be useful in the magnetic energy conversion technologies and biomedical sciences which lie far beyond those achievable with traditional magnetic materials.     

Aqueous dispersions of magnetite nanoparticles complexed with copolyether dispersants: experiments and theory

Magnetite (Fe3O4) nanoparticles have been synthesized and complexed with carboxylate-functional block copolymers, and then aqueous dispersions of the complexes were investigated as functions of their chemical and morphological structures. The block copolymer dispersants had either poly(ethylene oxide), poly(ethylene oxide-co-propylene oxide), or poly(ethylene oxide-b-propylene oxide) outer blocks, and all of them had a polyurethane center block that contained pendent carboxylate groups. The complexes were formed through interactions of the carboxylates with the surfaces of the magnetite nanoparticles. The magnetite cores of the magnetite-copolymer complexes were near 10 nm in diameter, and the particles were superparamagnetic. Complexes with mass ratios of polymer to magnetite varying from 50:50 to 85:15 were studied. One of our objectives is to design complexes that form stable dispersions of discrete particles in water, yet that can be actuated (moved together) upon exposure to a uniform magnetic field. DLVO calculations that accounted for magnetic attractive interparticle forces, as well as van der Waals, steric, and electrostatic forces are presented. Compositions were identified wherein a shallow, attractive interparticle potential minimum appears once the magnetic term is applied. This suggests that it may be possible to tune the structures of superparamagnetic nanoparticle shells to allow discrete dispersions without a field, yet weak flocculation could be induced upon exposure to a field.     

热处理对[NiFe/Cu/Co/Cu]_n多层纳米线磁性能的影响

采用双槽控电位法在阳极氧化铝(AAO)模板中制备了有序均一的[NiFe/Cu/Co/Cu]n多层纳米线阵列,并在不同温度下进行了热处理.利用X射线衍射(XRD)对热处理前后多层线的晶体结构进行了测试.考察了不同退火温度对多层线矫顽力、剩磁比、巨磁电阻(GMR)效应、磁灵敏度的影响.随热处理温度升高,多层纳米线中磁性微晶晶型取向越来越明显,晶体结构更均匀;多层纳米线的矫顽力和剩磁比先增大后减小.300°C下多层纳米线矫顽力达到最大值,GMR最大值可达59%,对应的磁电阻灵敏度(SV)为0.233%Oe-1.     

Self-assembly of poly(N-isopropylacrylamide)-carrying microspheres into two-dimensional colloidal arrays

Droplets containing polymer particles were deposited on a substrate. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel and particles with PNIPAM graft chains on the surface self-assembled into a two-dimensional (2-D) superlattice when their dilute dispersions were dried on substrates. The capillary force between the particles induced ordered array formation during water evaporation. The presence of a PNIPAM layer on the particle surface gave the particles steric stability during ordered array formation. By grafting PNIPAM chains on particle surfaces by living radical polymerization, we successfully controlled the structural patterns of the colloidal arrays. These, controllable, 2-D colloidal arrays were generated on various substrates upon air-drying.     

Polychlorinated biphenyls (PCBs) detection in milk samples by an electrochemical magneto-immunosensor (EMI) coupled to solid-phase extraction (SPE) and disposable low-density arrays

An electrochemical immunosensor for polychlorinated biphenyl (PCB) detection based on graphite screen-printed low-density arrays and on magnetic beads is reported.The immunological reaction for the detection of PCBs is based on a direct competitive assay using alkaline phosphatase (AP) as enzymatic label. After the immunochemical recognition, the modified magnetic beads are captured by a magnet on the surface of the graphite working electrode. The electrochemical detection is thus achieved through the addition of the AP substrate (α-naphthyl-phosphate).Two different antibodies (sIgG anti-PCB28 and rIgG anti-PCB77) were tested and compared in terms of sensitivity and ability to recognise different congeners. The developed electrochemical magneto-immunosensor (EMI) was successfully combined with solid-phase extraction (SPE) for the analysis of PCBs in milk samples. In spiked samples a recovery of 80% was obtained. The proposed strategy offers great promise for rapid, simple, cost-effective, and on-site analysis of clinical, food and environmental samples, considering also that low-density arrays allow the simultaneous analysis of different processed samples.     

On-chip magnetic bead microarray using hydrodynamic focusing in a passive magnetic separator

Implementing DNA and protein microarrays into lab-on-a-chip systems can be problematic since these are sensitive to heat and strong chemicals. Here, we describe the functionalization of a microchannel with two types of magnetic beads using hydrodynamic focusing combined with a passive magnetic separator with arrays of soft magnetic elements. The soft magnetic elements placed on both sides of the channel are magnetized by a relatively weak applied external magnetic field (21 mT) and provide magnetic field gradients attracting magnetic beads. Flows with two differently functionalized magnetic beads and a separating barrier flow are introduced simultaneously at the two channel sides and the centre of the microfluidic channel, respectively. On-chip experiments with fluorescence labeled beads demonstrate that the two types of beads are captured at each of the channel sidewalls. On-chip hybridization experiments show that the microfluidic systems can be functionalized with two sets of beads carrying different probes that selectively recognize a single base pair mismatch in target DNA. By switching the places of the two types of beads it is shown that the microsystem can be cleaned and functionalized repeatedly with different beads with no cross-talk between experiments.     

Continuous production of water dispersible carbon-iron nanocomposites by laser pyrolysis: application as MRI contrasts

Carbon encapsulated iron/iron-oxide nanoparticles were obtained using laser pyrolysis method. The powders were processed to produce stable and biocompatible colloidal aqueous dispersions. The synthesis method consisted in the laser decomposition of an aerosol of ferrocene solution in toluene. This process generated, in a continuous way and in a single step, a nanocomposite formed by amorphous carbon nanoparticles of 50-100 nm size in which isolated iron based nanoparticles of 3-10 nm size are located. The effect of using different carriers and additives was explored in order to improve the efficiency of the process. The samples after purification by solid-liquid extraction with toluene, were oxidised in concentrated nitric acid solution of sodium chlorate, washed and finally ultrasonically dispersed in 1 mM tri-sodium citrate solutions. The dispersions obtained have hydrodynamic particle size less than 150 nm and are stable in the pH range of 2-11. Finally the shortening of the transversal relaxation time of water protons produced by the dispersed particles was studied in order to test the feasibility of these systems to be traced by magnetic resonance imaging techniques.     

Fe-Co-Ni合金纳米线有序阵列的模板合成与磁性

以二次阳极氧化的氧化铝膜为模板，用电化学沉积的方法成功地合成了Fe-Co-Ni三组份有序纳米线阵列.扫描电子显微镜（SEM）和透射电子显微镜(TEM)观察表明纳米线表面光滑、有序、高长径比；磁性测量表明,其矫顽力较同组份的膜材料有较大的提高.将样品在惰性气体氛围中不同温度下退火，随着退火温度增加，其纵向矫顽力有一个极值，而对应的横向矫顽力没有类似的变化，关于这一现象的机理,本文进行了初步的讨论. 图5参15     

Assembly of multicellular constructs and microarrays of cells using magnetic nanowires

An approach is described for controlling the spatial organization of mammalian cells using ferromagnetic nanowires in conjunction with patterned micromagnet arrays. The nanowires are fabricated by electrodeposition in nanoporous templates, which allows for precise control of their size and magnetic properties. The high aspect ratio and large remanent magnetization of the nanowires enable suspensions of cells bound to Ni nanowires to be controlled with low magnetic fields. This was used to produce one- and two-dimensional field-tuned patterning of suspended 3T3 mouse fibroblasts. Self-assembled one-dimensional chains of cells were obtained through manipulation of the wires' dipolar interactions. Ordered patterns of individual cells in two dimensions were formed through trapping onto magnetic microarrays of ellipsoidal permalloy micromagnets. Cell chains were formed on the arrays by varying the spacing between the micromagnets or the strength of fluid flow over the arrays. The positioning of cells on the array was further controlled by varying the direction of an external magnetic field. These results demonstrate the possibility of using magnetic nanowires to organize cells.     

Long-Term Colloidally Stable Aqueous Dispersions of ≤5 nm Spinel Ferrite Nanoparticles

Applications in biomedicine and ferrofluids, for instance, require long-term colloidally stable, concentrated aqueous dispersions of magnetic, biocompatible nanoparticles. Iron oxide and related spinel ferrite nanoparticles stabilized with organic molecules allow fine-tuning of magnetic properties via cation substitution and water-dispersibility. Here, we synthesize≤5 nm iron oxide and spinel ferrite nanoparticles, capped with citrate, betaine and phosphocholine, in a one-pot strategy. We present a robust approach combining elemental (CHN) and thermal gravimetric analysis (TGA) to quantify the ratio of residual solvent molecules and organic stabilizers on the particle surface, being of particular accuracy for ligands with heteroatoms compared to the solvent. SAXS experiments demonstrate the long-term colloidal stability of our aqueous iron oxide and spinel ferrite nanoparticle dispersions for at least 3 months. By the use of SAXS we approved directly the colloidal stability of the nanoparticle dispersions for high concentrations up to 100 g L⁻¹.